1. Field of the Invention
The invention relates to a back light module, more particularly to a back light module for large LCD device.
2. Description of the Prior Art
Among all kinds of flat display devices, a liquid crystal display (LCD) device with low electrical power consumption, low voltage operation, thin thickness and light weight, is widely used in nowadays. But an additional light module is needed for LCD device to show figures or pictures on the screen under the dark circumstance because liquid crystal does not have the character of light-emitting itself. Because most of the power (about 70%) is consumed by the light module for notebook, it is very important how to increase the illumination of light module or get the same illumination by using the lower power light source for upgrading the efficiency of light module and decreasing the consumed power.
Please refer to FIG. 1, a cross-sectional view of conventional structure of back light module 110 for large LCD device is shown. It comprises a light source 112; a light guide plate (LGP) 114; a reflector 116; a diffusor sheet 118; and a diffusor sheet 120. The light source 112 can be constituted by a row of cold cathode fluorescent lamp (CCFL) or an array of light emitting diode (LED). The material of light guide plate 114 is usually PMMA or plastic division with TiO2 diffusion grains whose total volume is between about 50%˜80% of light guide plate. The light rays are well distributed when they are collided with the TiO2 diffusion grains. The reflector 116 is placed below the light guide plate 114 to reflect the downward light rays back into the light guide plate 114 for increasing the illumination of back light module 110. Besides, there are sequentially diffusor sheets 118 and prism sheets 120 on the light guide plate 114 for the light rays emitted from the light-emitting surface of light guide plate 114 can be further well distributed into the panel of liquid crystal device (not shown). The numbers of diffusor sheets and prism sheets are decided on the design of back light module 110.
The wave nature of light is well known now, so the obvious “interference” phenomenon is observed in the bottom surface of light guide plate 114 for the light rays emitted from the light source 112, as shown in FIG. 2. Sometimes the light rays meet in step (or in phase; constructive interference), sometimes they meet out of step (or out of phase; destructive interference), and sometimes they meet partially in step. When the light rays meet in step, they add together due to constructive interference and a bright fringe is displayed on the screen. In areas where the light rays meet totally out of step, they will subtract from each other due to destructive interference and a dark fringe will appear in the bottom surface of light guide plate 114. FIG. 3 is the enlarged view of the section A in FIG. 2. Similarly, the obvious “interference” phenomenon is still observed when the light source is constituted by an array of light emitting diode (LED).
Besides, a cold cathode fluorescent lamp (CCFL) is a line light source and a light emitting diode (LED) is point light source, so the illumination of light rays emitted from the above light source decreases when they travel longer distances. The maximum illumination on the section of light guide plate above the light source is got because the distance between the above both is shortest and the illumination on other section of light guide plate is smaller because the distance between the above both is longer. Therefore, a problem that the illumination on the bottom surface of light guide plate not well distributed is produced.